Electron transfer, activation control dissociative

Further experimental studies involved the determination of the rate constant of the reaction of several alkyl halides with a series of electrochemically generated anion radicals so as to construct activation driving force plots.39,40,179 Such plots were later used to test the theory of dissociative electron transfer (Section 2),22,49 assuming, in view of the stereochemical data,178 that the Sn2 pathway may be neglected before the ET pathway in their competition for controlling the kinetics of the reaction. 

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

Pulse radiolysis studies show that the rapid reduction of the type 1 centers is followed by a partial reoxidation as an electron is transferred to the type 2 Cu centers. The extent of reoxidation is determined by the relative redox potentials of the two centers. In the presence of nitrite the reaction (Equation (4)) proceeds to completion and the type 1 center becomes fully reoxidized. Typical values for Atet for both green (AcNiR) and blue (AxNiR) at pH 7 are 1.4 x 10 s , which decreases by 50% in the presence of nitrite. The rapid rate of reduction of the type 1 center was unaffected by pH but Atet showed a marked pH dependence and at pH 6 the rate was 2 X 10 s and was unchanged by nitrite. Curves for the pH Atet dependence were essentially the same as the pH activity curve suggesting that two proton dissociations with values of 5 and 7 play an important role in both processes. Studies with AxNiR mutants Asp98Ala and His255Ala showed that these residues in wild-type NiR control the rate of electron transfer from the type 1 to the type 2 center by the formation of an H-bonding network ... 

The homogeneous outer sphere electron transfer reactions in solution occur at a rate that is noticeably Icj er than the diffusion rate. This peculiar behaviour has been explained through a three-step mechanism formation of a precursor complex from the separated reactants, actual electron transfer within this complex to form a successor complex and dissociation of the latter complex into separated products. The reaction rate is usually controlled by the electron transfer step, this step being governed by the Franck-Condon principle. This principle is embodied in classical electron transfer theories using an activated-complex formalism in which the electron transfer occurs at the intersection of two potential energy surfaces, one for the reactants and the other for the products. This implies that the second step necessarily involves the reorganization of the solvent before and after the electron transfer itself is produced. So, it is obvious that solvent must play an essential role in the rate of electron transfer reactions in solution. 

The exchange mechanism is available also for triplet-triplet energy transfer between donor and acceptor molecules in contact [37]. It has been studied in rigid solvents by phosphorescence emission, and in fluid solvents by flash absorption spectrophotometry (see Section 4.4.3.2). The results [37,a] show that for exothermic processes the rate constant in fluid solvents approximates to the encounter-controlled value. There are cases where it is smaller, possibly because of an activation requirement, which may be expected if orbital interactions are involved, or if activated vibronic interactions are required to make the energy transfer electronically feasible, or if steric factors prevent favourable electronic interactions. Another possibility is that in solvents of low viscosity the collision complex may dissociate before there has been enough time for the electron-exchange to occur this would account for the case of valerophenone -f- 2,5-dimethyl-2,4-hexadiene, where in a series of solvents k /ko falls well below unity as the viscosity is decreased [37,c]. 

---